1. Field of the Invention
The invention relates to a motor control device, an image forming apparatus, a motor control method, and a computer-readable storage medium.
2. Description of the Related Art
As an image processing apparatus such as a printer, a facsimile, a scanner, a copier, and an MFP thereof, there is known an electrophotography image forming apparatus for forming an electrostatic latent image on a surface of a photosensitive drum that is an image carrier, making the electrostatic latent image on the photosensitive drum to a visible image by developing the electrostatic latent image with toner and the like that are a developer, transferring the developed image onto a recording sheet (also called a sheet, recording medium, and recording material) by a transfer device and causing the recording sheet to carry the image, and fixing a toner image on the recording sheet by a fixing device using pressure, heat, and the like.
Further, there is known a so-called inkjet image forming apparatus for forming an image by depositing ink as a liquid on a recording sheet while conveying the recording sheet using a device including a recording head composed of a liquid ejection head.
In the image forming apparatuses described above, it is known that a DC motor (direct current motor, hereinafter, also simply called a motor) is used as a drive unit of a sheet conveying unit and the sheet conveying unit is driven by executing feedback control by detecting the rotation speed of the DC motor so that the rotation speed becomes a set target speed.
Although a DC motor generally has small average power consumption as compared with a conventionally used stepping motor, it has a large maximum current. Accordingly, when some disadvantage and the like occur and a load becomes large, a current flowing to the DC motor increases and the DC motor generates excessive heat and a danger arises in that the DC motor is damaged depending on circumstances.
In particular, when the DC motor is applied with an excessive load and placed in a lock state (state that the DC motor cannot be rotated and is stopped), there is a possibility that a large amount of a current flows to a switching element such as a field-effect transistor (FET) and the like in a control circuit of the DC motor and the switching element is damaged.
To overcome the problem, there is proposed a technology for detecting a lock state from a pulse change amount of a signal from a Hall element (magnetic sensor) of a DC motor, and the like and cutting off the output of the DC motor and a technology for avoiding a lock state from being erroneously detected from a state of a DC motor or a load driven by the DC motor.
For example, Japanese Patent Application Laid-open No. 2002-347296 discloses a matter as to a DC motor used to drive an ink carrier of an inkjet printer. According to a technology of Japanese Patent Application Laid-open No. 2002-347296, first, each time the duty of a pulse signal is controlled at each predetermined time, whether or not the duty of the pulse signal becomes a maximum value is determined. The number of times the duty becomes the maximum value is counted by a counter and the number of times counted by the counter is increased until the counted value of the counter becomes a predetermined value without stopping the DC motor even if the duty has become the maximum value. Thereafter, when the count value of the counter has reached the predetermined value, it is determined that the DC motor is in the lock state (in which the DC motor cannot rotate and is stopped), and a voltage applied to the DC motor is interrupted.
Further, for example, Japanese Patent Application Laid-open No. 2004-324105 discloses to detect the engine stop of a vehicle and to change a determination threshold value used to determine whether or not a window glass is sandwiched when a battery voltage has reached a predetermined lower limit value (when a voltage stabilizing time has passed). From the technology, there is disclosed a device for avoiding an erroneous detection of lock of a DC motor for opening/closing a window glass.
In an image forming apparatus, it is necessary to place a DC motor in a state in which the DC motor is stopped while holding the rotation position of the motor to provide a sheet with a sag while the sheet being conveyed (hereinafter, called a position hold state) in addition to that the DC motor is subjected to a state of ordinary sheet conveying control (acceleration/constant speed/deceleration control) (hereinafter, called an ordinary rotation state).
However, the conventional technologies for avoiding the erroneous detection of lock have a problem described below. That is, the conventional technologies for avoiding the lock erroneous detection do not execute control by discriminating whether the DC motor is in an ordinary rotation state or in a position hold state. Accordingly, a problem arises in that when a motor is controlled to a position hold state, since it is erroneously detected that the motor is in a lock state and the output of the motor is cut off, the rotation position of the motor (that is, a sheet position) cannot be held.
Therefore, there is a need to avoid an erroneous detection of lock of a motor in a state that stop control is executed while holding the rotation position of the motor and to stop the motor while holding the motor position.